The gas content of galaxies has been measured commonly via CO emission (a tracer of H\(_2\) ) out to \(z\)\(\sim \) 3 [1]} and it is found to increase with redshift, reflecting the increase in sSFR [2]}. The measurement of the gas content of galaxies out to higher redshift is challenging and has not been measured for large samples of typical galaxies prior to ALPINE. Low-\(J\)  CO emission becomes increasingly more difficult to observe at high redshifts because of surface brightness dimming, lower metal content of the galaxies, and redshift itself. Furthermore, the increasing cosmic microwave background temperature with redshift decreases the contrast of the line [3]}. On the other hand, the conversion of high-\(J\)  CO transitions to CO(1-0) requires knowledge of the CO excitation stage and the gas density. In addition, even with state-of-the-art millimeter facilities such as VLA or NOEMA, a detection of high-\(J\)  CO emission becomes unfeasible for a large sample of galaxies.
For understanding the molecular gas content of high redshift galaxies, other tracers of molecular gas have to be explored. Among these are the dust mass inferred from an SED fit to the far-IR emission [4]}, [5]}, [6]}, the cold dust continuum measured in the Rayleigh–Jeans (RJ) tail regime at \(\lambda _{\rm rest} \approx 850\,{\rm {m}}\)  [7]}, [8]}, and the empirically measured relation between C\(^+\)  and molecular gas mass [9]}, [10]}, [11]}. Furthermore, using an estimate of stellar, dust, and dark matter mass inside a given radius, dynamical mass estimates from kinematics can be used to constrain gas masses [12]}, [13]}.
The ALPINE sample provides galaxies with the necessary far-IR continuum and C\(^+\)  line luminosity measurements; C\(^+\)  kinematics; and more importantly, far-IR and C\(^+\) -LFs, to undertake the first comprehensive measurement of the gas content of main-sequence galaxies in the post-reionization phase at \(z=\)  4–6.
